Conventional bolt assemblies for securing roof or wall supports to the rock structure of a mine often incorporate an adhesive anchoring means that is inserted, along with an elongate threaded bolt, into a hole that has been bored into the rock structure. The hole and the bolt each generally have a length of about five to eight feet. The adhesive anchoring means consists generally of a thin-skinned resin capsule having a length of about twelve to twenty-four inches. The greater the length of the capsule, the greater the holding power of the adhesive anchoring means. The capsule contains unmixed quantities of a quick-setting adhesive resin and a catalyst hardener resin. A portion of the bolt extends from the hole and passes through an opening in the support. This portion of the bolt is then threadedly engaged with a nut to suspend the support from the bolt.
Means are provided to limit the threaded advance of the nut on the bolt. Rotation and upward thrusting of the nut, for instance by a power tool with an adapter such as a roof bolt dolley, thus causes the bolt to rotate within the bore hole and ruptures the resin capsule. The upper portion of the bolt is typically pointed to assist in rupturing the capsule. The resin materials are then mixed by the rotating bolt in order that they flow uniformly about the hole and the bolt. Once the resin materials have been mixed, rotation of the nut and bolt is ceased and the resin mixture is allowed to harden within the hole. The hardened mixture serves to strengthen the surrounding rock structure as well as to anchor the bolt within the hole.
When the resin mixture has hardened, an excessive torque is applied to the nut to overcome the advancement limiting means and allow further threaded advance of the nut along the bolt to tighten the roof or wall support to the rock structure.
One example of a conventional bolt assembly is described in Canadian patent 1,187,709 (Clark et al.) where the limiting means is located on the nut and consists of a frangible dome. Other examples are described in U.S. Pat. Nos. 3,877,235 (Hale) and 4,023,373 (Hipkins) where the limiting means is a frangible disk or internal nut, U.S. Pat. No. 3,940,941 (Libert et al.) where the limiting means is a discontinuity in the thread of the bolt, and U.S. Pat. No. 3,979,918 (Vidler) where the limiting means is a discontinuity in the thread of the nut.
Such conventional bolt assemblies have several disadvantages. Those that incorporate frangible elements tend to shower fragments of those elements into the body of the power tool adapter being used to rotate the nut thus filling the adapter with fragments. Those that incorporate a discontinuity in the thread of the bolt do not permit a second nut to be threaded onto the bolt to attach screening. Similarly, those that incorporate a discontinuity in the thread of the nut tend to damage the thread of the bolt so that a second nut cannot be attached. Such discontinuities in the thread of the nut may also cause broken pieces of the nut or the bolt to shower into the workings of the power tool. Finally, the limiting means of most conventional bolt assemblies tend to be relatively expensive to manufacture.
The object of the present invention is to provide an advancement limiting means that may be manufactured in a manner that is more cost-effective than the examples disclosed by the prior art. The limiting means will not be frangible so as to hinder the use of the power tool adapter (e.g. roof bolt dolley), and the limiting means will not require or cause the thread of the bolt to become damaged. In addition, the invention will provide an advancement limiting means that may be easily varied in its manufacture such that its overcoming torque may be increased or decreased according to specific applications.